[PowerPC] Fix the liveins for ppc-expand-isel pass

Summary:
In the ppc-expand-isel pass, we use stepForward() to update the
liveins, this function is not recommended, because it needs the
accurate kill info.

This patch uses the function computeAndAddLiveIns() to update the
liveins, it's the recommended method and can fix the liveins bug for
ppc-expand-isel pass..

Reviewed By: efriedma, lkail

Differential Revision: https://reviews.llvm.org/D78657